Classification of Igneous Rocks

Dr. Ahmad Redaa

2024-09-25

Bases of Igneous Rock Classification

Issues with Terminology

Terms in Literature

The IUGS Classification

Plotting on a Triangular Diagram

Source: Winter, J. (2014)

Steps to Plot 70% X, 20% Y, and 10% Z

1. Understand the Axes of the Triangular Diagram

2. Plot the 70% X Value

3. Plot the 20% Y Value

4. Locate the Point

5. Verify the Z Value

Final Result

Classification Based on Mineral Content

Source: https://www.britannica.com

Classification of The Plutonic Igneous Rocks

Source: Winter, J. (2014)

Classification of Volcanic Rocks

Source: Winter, J. (2014)

Chemical Classification of Volcanic Rocks

Source: Winter, J. (2014)

Chemical Classification

  1. Silica Saturation
  2. Alumina (Al2O3) Saturation
  3. Alkaline/Subalkaline Rocks

Silica Saturation

If a magma is oversaturated with respect to silica, then a silica mineral such as quartz, cristobalite, tridymite, or coesite should precipitate from the magma and be present in the rock. On the other hand, if a magma is undersaturated with respect to silica, a silica mineral will not precipitate and thus will not be present in the rock. This concept divides rocks into three categories:

1. Silica Undersaturated Rocks

These rocks contain minerals that, in general, do not coexist with quartz. Such minerals include:

These minerals indicate a silica-undersaturated rock. When calculating a CIPW norm, silica-undersaturated rocks will contain normative nepheline and/or leucite.

Note

The CIPW Norm (Cross, Iddings, Pirsson, Washington Norm) is a method used to calculate the idealized mineral composition of an igneous rock based on its chemical analysis. It provides a way to standardize and classify rocks by computing a theoretical set of minerals (the norm) that would form from a complete crystallization of the rock’s magma under dry conditions at low pressure.

2. Silica Oversaturated Rocks

These rocks can be identified as those that do not contain any of the minerals listed above. When calculated in the CIPW norm, silica-oversaturated rocks will contain normative quartz.

3. Silica Saturated Rocks

Silica-saturated rocks contain just enough silica that neither quartz nor the silica-undersaturated minerals appear. In the CIPW norm, these rocks typically contain olivine or hypersthene + olivine, but lack quartz, nepheline, and leucite.

Alumina (Al₂O₃) Saturation

Source: https://geologyhub.com

1. Peraluminous Rocks

2. Metaluminous Rocks

3. Peralkaline Rocks

Alkaline vs Subalkaline Rocks

Source: https://www2.tulane.edu

Rocks can be classified as alkaline or subalkaline based on their alkali content relative to silica, using an alkali vs. silica diagram.

1. Alkaline Rocks

2. Subalkaline Rocks

Note: Very alkaline rocks, which plot well above the dividing line in the alkali vs. silica diagram, are usually silica undersaturated.

Classification of Pyroclastic Rocks

Source: Winter, J. (2014)

Fragment Size Classification

Key IUGS Requirements

Types of Pyroclastic Rocks

Mixed Pyroclastic and Sedimentary Rocks

Special Types of Pyroclastic Deposits

Other Ignous Rocks

Source: Winter, J. (2014)